You probably just need to reload the sound driver, it is a common problem with windows for drivers to stop working where the hardware is just fine. It is really good to ask your real life friends and see if any of them know how to troubleshoot operating system problems before you start buying new computers or computer parts. I have almost never seen hardware (other than a hard drive) fail, but every few months a driver fails due to an operating system error on one of our computers. With someone like me, fixing them costs absolutely nothing. Sound and video drivers seem to give me the most issues, they frequently cease working for me in Windows XP (every few months to a year). If that does not fix it, I always re-install the operating system ENTIRELY before buying any hardware components.
Now if your really stumped, take it into the local Best-Buy and just let the Geek Squad deal with it. Their repair bills tend to be steep, but if you only need to do it once every few years, then it is not such a big deal. Now with that said, not every Geek-Squad is reputable, but they will give you a good estimate before repairing. There was a study done on them and some would charge over $200 for a simple cord that was not plugged in, saying something like, it needs a new motherboard or power supply when it clearly did not!